Abrasive disk



Patented Apr. 9, 1946 ABRASIVE DISK Robert W. Hackett, East Braintree,Mass" assignor to Abrasive Products, Inc.,

a corporation of Massachusetts tree, Mass.,

South Brain- Application October 8, 1941, Serial No. 414,158. (Cl.51-301) 6 Claims.

This invention relates to abrasive sheet material and more particularlyto flexible abrading disks of the type extensively used in grindingoperations'on metal.

Animal glue adhesives, which are widely used in making abrasive sheetproducts as a means for attaching the abrasive grit, are subject tocertain drawbacks when used in the manufacture of abrasive metalgrinding disks, especially those having fairly stiff sheet backingmaterial for use in so-called heavy duty" wor In the first place, thesedisks are rotated at very high speeds, which develop high surfacetemperatures at the point of abrasion. These temperatures naturally havea pronounced effect on the material which bonds the abrasive grit to thebacking. When such disks are surfaced with conventional animal glueadhesive coats, the glue, being thermoplastic and having little or noheat resistance, tends to soften and become tacky after short abradinguse at high rotating speeds. The softened adhesive then attracts andholds dust which arises from the abrading operation. Accumulation ofsuch is a very real problem, as such accumulation progressivelydiminishes the cutting power of the abrasive grit and the life of thedisk is measurably shortened despite the fact that the grit remainspotentially abradant and is still satisfactorily bonded to the sheetbacking. The life of the disk Because of the drawbacks of glue a fewmanufacturers have resorted to substitute adhesives and such asubstitute adhesive for binding the abrasive grit has been suggested inU. S. patent to Redman No. 2,176,942. This particular patent describesthe use of a heat hardened resin binding coat allegedly flexibilized bycertain modifying agents, for attaching the grit to the backing. Butdisks, especially those manufactured for use in metal grindingoperations, in which the abrasive is bonded to the backing by suchsubstitute dust on grinding wheel disks adhesives are much inferior fromthe standpoint of cutting power. Unlike glue, resinous materials have apronounced tendency to coat over and varnish the abrasive grit. This isdue to the fact that coatings of such resinous materials do not setquickly. After abrasive grains have therefore been sprinkled on acoating of the liquid syrupy resinous material they have a tendency tobury themselves by sinking down into the coating before it sets. Thisresults in the resinous material coating over the tops of many of thesmaller particles and often of all the grit where festooning is utilizedduring drying. Where the grit is buried, substantial loss of freecutting power results. Glue, on the other hand, is customarily appliedhot to the backing and skims over to an appreciable extent even beforethe then ceases to be measured by the integrity of A second objection tothe use of such glue in.

abrasive disks, especially where relatively stiff, tough, backings areused, is that the glue, the term glue being h'erein used to mean animalglue, in its ordinary form, is hygroscopic and subject to change inatmospheric conditions When used as a binder for grit, therefore, inmetal grinding flexible disks of relatively small diameter, themanufacturer is confronted with a curling tendency resultlng'from theeffects of change in the amount of moisture present in the surroundingatmosphere, In many instances this curling tendency necessitates aspecial steaming operation after drying of the completed sheet and eventhen, subsequent conditions of storage may later cause such curling ofthe disk as to ruin it because the grinding machine.

buried. Glue has abrasive grit can be spread and sets longbefore theweight of the grit can function to sink the In other words the longer aand unset after application liable is the grit to become always beenconsidered the most satisfactory abrasive binder because of itscharacteristics I spread will not become embedded in the glue coatingand that the ends of the particles will be exposed and uncovered,thereby providing a powerful cutting surface. I

Additionally, resinous binding coats are inferior to glue in qualitiesof bility a d toughness, especially when they do not grit into the glue.binder remains wet of a grit, the more include plasticizing agents andcatalysts which, i

if used, complicate their formulation.

For the above reasons, retention of a glue makingor bonding coating is,in the last analysis, highly desirable in the manufacture of thesedisks, especially when one'considers that the simplicity and cheapnessof a glue bond are universally recognized as unsurpassed. Nevertheless,unless dust accumulation be overcome, itsadvantages are, for practicalpurposes, nullified so far as prolonging the useful lifeof the disk isconcerned.

which insure that the grit when strength, resilience, fiexi-Accordingly, it is an object of this invention to overcome the dustaccumulation difilculty arising glue adhesive bonding surface which isnot subject to softening as a 'result of the operating heat, theclogging difllculty Heretofore infusible resins skilled in the art asmay be overcome. have been discarded by those having a practical use insuch overlying coats because, by their very nature, they involve heattreatment to harden and render them dry, and

glue cannot be subjected to the temperatures necessary to perform thisoperation without invariably making the-glue brittle after cooling,impairing its toughness and strength, and destroying the very qualitieswhich are responsible for its choice. Experiencedabrasive manufacturersaccept the principle that heat treatment of set glue during manufacturewill destroy its superiority as a binder and render it commerciallyunacceptable because of its tendency to crack and break up.

I have found, howev r, that if a heat-hardenable water-solublephenol-aldehyde resin is spread in aqueous solution overa glue makingcoat, the composite material may be baked at the necessary temperatureto'harden the resinous top coat without rendering the glue coat brittleafter cooling, and, in fact with noticeable improvement in the strengthand toughness of the glue making coat.

This phenomenon is responsible for my ability to make a vastly improvedglue bonded abrading disk, the life of whichis measurably lengthened dueto the combined presence of a highly adhesive tough and flexible gluemaking coat plus a heat hardened hard, dry, heat resistant top surfacecoat, but with the glue in no worse condition for adhesive purposes thanif it had not been subjected to heat. 4 a

My explanation for this phenomenom is that by applying the resin in asolvent which is a mutual solvent for glue, i. e., water, and by using aphenol-aldehyde resin, the resin lsto some degree carried into andpenetrates the glue coat and, because of the inherent characteristics ofthe resin, the glue coat, is, in a sense, conditioned by the resinousingredients so treatment involved in the subsequent baking withoutdeveloping brittleness.

In accordance with this explanation, and as I have found in practice, itis only the aqueous spread water-soluble, phenolic condensation resinswhich have this heat protective brittlenesspreventing capacity whenapplied over a glue coat.

Secondly, I have discovered that certain watersoluble phenol-aldehyderesins not only act to condition the glue for surviving the heattreatment, but in addition render thev glue less hygroscopic andconsequently less subject to curling after drying. Thephenol-formaldehyde resins are especially effective for this purpose,and I attribute this action to the free formaldehyde content which ispresent in water-soluble phenolformaldehyde resins. Because of theaqueous spreading, the resin penetrates into the aqueous that itsurvives the heat vent varnishing over wettable glue, carrying with itthe free formaldehyde content which. in accordance with knownprinciples, cures and conditions glue to render it moisture resistant.While the phenol-formaldehyde resins are especially efficacious, any ofthe phenol-aldehyde resins which have a free formaldehyde contentrunning from approximately 3% to 5% would be suitable to accomplish thisobject.

To describe more fully an embodiment of my invention, a conventionalfiberboard or combina tion paper and cloth flexible backing otherwisesuitable for the purpose making coat of a hot conventional glueadhesive. Abrasive grains are then spread over the glue as it sets, andbefore complete drying of the glue, an aqueous solution of awater-soluble phenolformaldehyde resin is spread, as by a doctor bladeor roll, over the abrasive coated surface. Because the glue haspartially set, it takes up the aqueous applied solution, with the resultthat the resin penetrates into the glue coating and tends to preorburyin of the grit, especially if the precaution of squeezing off excesssolution is taken, as by passing the material through pressure squeezerolls. After application of the top coating, the composite material isallowed to dry. Thereafter the-sheet material may be baked andsubsequently cut into disk shape.

Such a disk is illustrated in the accompanying drawing in which Fig. 1is a. perspective view of an abrasive disk bonding abrasive grit 8 tothe backing 2 and a protective coat 6 initially water-soluble i phenolicresin,

of, the heat-hardened I but the whole sheet having been baked to hardenthe coat 8.

While thetemperature and time of baking will vary depending uponparticular conditions of grit size, coating thickness and other factorswith which those in the art are familiar, as well as upon the particulartype of water-soluble phenolic resin being used in practice I have foundthat,

when using the water-soluble phenolic resin now available on the marketas a product of Catalin Corporation under the tradename Catabond No.

630, a temperature of the range of 200 F. over- While I mention thisparticular doubt there are other waternight is suitable. phenolic resin,no

soluble phenol-aldehyde condensation products having the samecharacteristics, elsewhere available, though this material seemsespecially suitable from the standpoint of its fre formaldehyde content,which runs from approximately 3-5%. The temperatures necessary forsecuring a permanent set may vary with the use of other similar resinsfrom room temperature upwardly to 250 F 'It is apparent that myinvention is applicable to other types of flexible abrasive sheets, forinstance all types. of sandpapers and abrasive cloths. and is availablefor equally advantageous use wherever a baked back coating is desired aswell as a baked workingsurface coat in connection with an abrasive sheethaving a glue bonded grit.

In its preferred form, an abrasive disk manufactured in accordance withthis invention presents a verypowerful cutting surface which is stableand not affected by heat; its surface remains hard and non-stickywithout tendencyto may be coated with a ruse, even when operatingagainst metal at speeds of 4500 linear feet per minute or higher, atwhich speeds temperatures 01' 750 F. to 1200 F. and

over are often developed on the abrading surface. At the same time,'thegrit is bonded to the backing by a tough flexible and highly adherentand coherent glue having at least as good adhesive characteristics asnormal glue decontinuous protective coat formed of the dried residuum ofan aqueous solution of an initially water-soluble phenol-aldehydecondensation resin overlying said making coat, surrounding and partiallyembedding said abrasive grit, penetrating said making coat and servingto maintain the flembility and toughness of said glue making coatdespite application of heat to said sheet in hardening said resin coat,said sheet presenting a working surface of said grit and said resincoat, which suriace tends to shed abrading dust even at the hightemperatures encountered in abrading operations.

2. The method of maintaining flexibility and toughness in a conventionalabrasive grit glue binder coat when submitted to a baking operationwhich comprises coating said binder. with a water-soluble heathardenable phenol-aldehyde resin in aqueous solution, drying saidresinous coating, and baking said resin to harden said resin to a hard,dry, heat resistant state.

3. The method 01' making a flexible abrasive sheet adapted for use inabrading operations developing high temperatures at the working surface,comprising afllxing abrasive grit to a suitable flexible sheet backingby a making coat of glue, applying in aqueous solution over said gluecoat before the glue has completely set, a watersoluble phenol-aldehydecondensation resin while allowing penetration of said resin in saidaqueous solvent into said glue making coat, and then subsequently bakingSaid sheet to 'set up said resin into a heat hardened, dry,heat-resistant coat, whereby said sheet will present a working surfaceof abrasive grit surrounded by said resin coat, the penetration of saidresin into said glue serving to maintain the flexibility and toughnessof said glue making coat despite said baking operation.

4. A baked flexible abrasive sheet having abrasive grit bonded to aflexible sheet backing by aflexible tough making coat of glue and anoverlying heat-hardened protective coat of an initially water-solublephenol-aldehyde resin, which sheet in its preparation has utilized themethod of claim 2 for maintaining the flexibility and toughness of theglue coat during the baking of the sheet to harden the resin,

5. A baked abrasive sheet having abrasive grit bonded to a flexiblesheet backing by a flexible tough making coat of glue and an overlyingheathardened protective coat of an initially watersolublephenol-aldehyde resin, which sheet has been prepared by the methodclaimed in claim 3.

6. The method of making a flexible abrasive sheet which comprisesaflixing abrasive grit to a flexible sheet backing with a glue adhesive,applying on the grit bearing'surface of said sheet an aqueous solutionof a' heat hardenable watersoluble phenol-aldehyde resin having a freeformaldehyde content, while allowing penetration of said resin in saidaqueous solvent into said glue adhesive and subsequently setting up saidphenolic resin to an infusible state.

ROBERT W. HACKE'IT.

